KR20060126906A - Spray polymerisation method - Google Patents

Abstract

The invention relates to a method for spray polymerisation of aqueous monomer solutions containing radically polymerisable monomers and at least 55 wt.- % water, for the production of cross-linked, water soluble polymers.

Description

Spray Polymerization Method

The present invention relates to an improved spray polymerization process for free radically polymerizable monomers.

Crosslinked water expandable polymers, also known as hydrogels or superabsorbents, can absorb aqueous fluids at several times their own weight. They are widely used in hygiene products such as diapers and sanitary napkins. These contain polymerized units of water-insoluble ethylenically unsaturated monomers such as carboxylic acids and amides. Water-insoluble polymers are obtained by incorporating crosslinking sites into the polymer. The degree of crosslinking determines the water solubility of the polymer as well as its absorption capacity.

A further criterion of a water expandable polymer is its extractable content. The extractables are washed when the water expandable additive polymer is in contact with the fluid, reducing the absorption capacity.

Patent application EP-A-0 348 180 describes a spray polymerization process for water absorbent resins. In this process, aqueous solutions of partially neutralized acrylic acid, crosslinkers and initiators are sprayed and polymerized in the air stream. The patent application teaches that the airflow should have a relative humidity of at least 30%. At lower relative humidity, the water present in the droplets will evaporate too quickly and the monomer will precipitate in the droplets and will no longer be available for polymerization, resulting in incomplete monomer conversion. A disadvantage of this method is that it requires removal of the product from a completely insulated or trace-heated vessel (otherwise the steam will condense on the product).

US 5,269,980 describes a process for producing polymer particles. In this method, the aerosol is generated from a polymer solution, a solution of prepolymerized monomers or a monomer solution, and these aerosol particles are dried / polymerized at temperatures in excess of 150 ° C. Polymerized aerosol particles are too small for use as water absorbent resins.

In patent application WO 96/40427, spray polymerization is carried out by spraying a monomer solution into a hot and essentially static atmosphere. At reduced pressures, the water content of the resulting polymer spheres is noticeably lowered, but the polymer particles have a rough surface. At elevated pressures, smooth polymer spheres are obtained.

The desired low residual water content should be able to be achieved without further work, for example without drying. For this reason, a low water content of less than 30% by weight was chosen for the monomer solution.

Patent application EP-A-0 816 383 teaches the preparation of hydrophilic and highly expandable hydrogels in fluidized bed apparatus. The polymer particles produced by the method described in this patent application have a diameter of 0.1 to 2 mm and have an onion skin structure. That is, the polymer grains present are sprayed with additional monomer solution and the monomer solution is then polymerized. This also gives a larger polymer particle than the above mentioned methods.

It is an object of the present invention to provide an improved spray polymerization process for the preparation of crosslinked water expandable polymers.

The inventors have finally found that crosslinked water-expandable polymers with improved properties with respect to absorbency and extractables are obtained by spray polymerization of monomer solutions having a water content of at least 55% by weight in an inert atmosphere.

The water content is preferably 60 to 95% by weight, more preferably 65 to 90% by weight, in particular 70 to 85% by weight.

Monomer solutions that can be used in the process of the invention contain, for example, at least a) free radically polymerizable monomers, b) crosslinkers, c) polymerization initiators and d) water.

The free radically polymerizable monomer a) is for example an ethylenically unsaturated C ₃ -C ₆ -carboxylic acid, its amides and esters with amino alcohols of the general formula (I).

Wherein R ⁴ is C ₂ -C ₅ -alkylene and R ¹ , R ² and R ³ are independently hydrogen, methyl, ethyl or propyl.

Examples of this compound are acrylic acid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, crotonic acid, maleic acid, maleic anhydride, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconic acid and fumaric acid. Alkali metal or ammonium salts of these acids, acrylamide, methacrylamide, crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminobutyl Acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate and dimethylaminoneopentyl methacrylate. Basic acrylates and methacrylates are used in the form of salts with strong inorganic acids, sulfonic acids or carboxylic acids or in quaternized form. X ^- anions in the compounds of formula (I) are inorganic or carboxylic acid residues or methosulfates, ethosulfates or halides from quaternizing agents.

Further free radically polymerizable monomers a) are alkali metal or ammonium salts of N-vinylpyrrolidine, acrylamidopropanesulfonic acid, vinylphosphonic acid and / or vinylsulfonic acid. Other acids can likewise be used for polymerization in unneutralized form or partially or 100% neutralized form. Useful free radically polymerizable monomers include N-vinylimidazolium compounds such as salts of N-vinylimidazole and 1-vinyl-2-methylimidazole or quaternized products, N-vinylimidazoline, 1- Further included are N-vinylimidazolines, such as vinyl-2-methylimidazoline, 1-vinyl-2-ethylimidazoline or 1-vinyl-2-n-propylimidazoline, all of which are likewise Used in polymerization as quaternized form or salt.

Further monoethylenically unsaturated sulfonic acids or phosphonic acids such as allylsulfonic acid, sulfoethyl acrylate, sulfoethyl methacrylate, sulfopropyl acrylate, sulfopropyl methacrylate, 2-hydroxy-3-acrylo Iloxypropylsulfonic acid, 2-hydroxy-3-methacryloylpropylsulfonic acid, allylphosphonic acid, styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid are likewise suitable. The monomers a) can be used alone or in admixture with each other.

Preferred monomers a) include acrylic acid, methacrylic acid and also alkali metal or ammonium salts of these acids, acrylamide and / or methacrylamide, further vinylsulfonic acid, acrylamidopropanesulfonic acid or mixtures of these acids, eg For example a mixture of acrylic acid and methacrylic acid, a mixture of acrylic acid and acrylamidopropanesulfonic acid or a mixture of acrylic acid and vinylsulfonic acid. These monomers may be copolymerized with each other in any ratio desired.

Preferred monomers a) further include mixtures of the abovementioned acids with their alkali metal or ammonium salts. For example, a mixture of acrylic acid and its alkali metal salts can be obtained by neutralizing acrylic acid with alkali metal hydroxides and / or alkali metal carbonates. The degree of neutralization is preferably 40% or more, more preferably 60 to 90%. In this regard, for example, 50% neutralization from a mixture of acrylic acid and sodium acrylate means that sodium acrylate and acrylic acid are present in a molar ratio of 50:50, while 75% neutralization means that sodium acrylate and acrylic acid are Molar ratio 75:25.

Particularly preferred monomers a) are acrylic acid, methacrylic acid, potassium salts of these acids and also mixtures thereof, for example mixtures of acrylic acid and potassium acrylate.

The polymerization of monomer a) takes place in the presence of one crosslinker b) or a combination of various crosslinkers.

Suitable crosslinkers b) are, for example, (meth) acrylic esters of polyhydric alcohols which may be alkoxylated with up to 100, usually up to 50 ethylene oxide and / or propylene oxide units. Suitable polyhydric alcohols are in particular C ₂ -C ₁₀ -alkanepolyols having ₂ to 6 hydroxyl groups such as ethylene glycol, glycerol, trimethylolpropane, pentaerythritol or sorbitol. Preferred crosslinkers are polyethylene glycol diacrylates and polyethylene glycol dimethacrylates, which are derived from polyethylene glycols (which can be regarded as ethoxylated ethylene glycols) with molecular weights 200 to 2000, respectively. Further usable crosslinkers b) include trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, ethylene glycol diacrylate, propylene glycol diacrylate, butanediol diacrylate, hexanediol diacrylate, hexanediol dimetha Acrylates or diacrylates and dimethacrylates of block copolymers formed from ethylene oxide and propylene oxide.

Useful crosslinkers b) include diallyl carbonates, allyl carbonates or allyl ethers of polyhydric alcohols which may be alkoxylated up to 100, usually up to 50 ethylene oxide and / or propylene oxide units, and polybasic Further included are allyl esters of carboxylic acids.

Allyl carbonate of the polyhydric alcohol follows the general formula (II).

Wherein A is a radical of a polyhydric alcohol which may be alkoxylated with up to 100, usually up to 50 ethylene oxide and / or propylene oxide units, n being the hydrogenity of the alcohol It is an integer of 2-10, Preferably it is 2-5.

Particularly preferred examples of such compounds are ethylene glycol di (allyl carbonate). Also suitable are polyethylene glycol di (allyl carbonates) derived from polyethylene glycols having a molecular weight of 200 to 2000.

Preferred examples of allyl ethers include polyethylene glycol diallyl ether, pentaerythritol triallyl ether or trimethylolpropane diallyl ether derived from polyethylene glycol having a molecular weight of 200 to 2000. It is also possible to use reaction products of ethylene glycol diglycidyl ether or polyethylene glycol glycidyl ether with 2 mol of allyl alcohol and / or pentaerythritol triallyl ether.

An example of a suitable allyl ester with a polybasic carboxylic acid is diallyl phthalate.

The monomers are generally copolymerized with each other in an aqueous solution of up to 45% by weight, preferably 5 to 40% by weight, more preferably 10 to 35% by weight, in particular 15 to 30% by weight, in the presence of the polymerization initiator c).

Useful polymerization initiators c) include all compounds which decompose into free radicals under polymerization conditions, for example peroxides, hydroperoxides, hydrogen peroxides, persulfates, azo compounds and redox catalysts. Preference is given to using water insoluble initiators. In some cases, it is advantageous to use mixtures of various polymerization initiators, for example hydrogen peroxide and mixtures of sodium peroxodisulfate or potassium peroxodisulfate. Mixtures of hydrogen peroxide and sodium peroxodisulfate can be used in any ratio. Useful organic peroxides include, for example, acetylacetone peroxide, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butyl perpivalate, tert-butyl peroxide Neohexanoate, tert-butyl perisobutyrate, tert-butyl per-2-ethylhexanoate, tert-butyl perisononanoate, tert-butyl permaleate, tert-butyl perbenzoate, di (2- Ethylhexyl) peroxydicarbonate, dicyclohexyl peroxydicarbonate, di (4-tert-butylcyclohexyl) peroxydicarbonate, dimyristyl peroxydicarbonate, diacetyl peroxydicarbonate, allyl perester, cumyl peroxyneode Decanoyl, tert-butyl per-3,3,5-tri-methylhexanoate, acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl peroxide and tert-amyl perneodecanoate The. Useful polymerization initiators c) include water-soluble azo initiators such as 2,2'-azobis (2-amidinopropane) dihydrochloride, 2- (carbamoylazo) isobutyronitrile, 2,2'-azobis -[2- (2'-imidazolin-2-yl) propane] dihydrochloride and 4,4'-azobis- (4-cyanovaleric acid) are further included. The polymerization initiator c) mentioned is used in conventional amounts, for example in amounts of 0.01 to 5% by weight, preferably 0.05 to 2% by weight, based on the monomers to be polymerized.

Useful initiators c) further include redox catalysts. In the redox catalyst, the oxidizing component is at least one of those described above per compound, and the reducing component is for example ascorbic acid, glucose, sorbose, ammonium or alkali metal bisulfite, sulfite, thiosulfate, hyposulfite, Pyrosulphites or sulfides, or metal salts such as iron (II) ions or silver ions or sodium hydroxymethylsulfoxylate. The reducing component in the redox catalyst is preferably ascorbic acid or sodium pyrosulfite. Based on the amount of monomer used for the polymerization, the reducing component of the redox catalyst is used at 1 · 10 ^-5 to 1 mol%. It is also possible to use one or more water soluble azo initiators instead of the oxidizing component of the redox catalyst.

The process of the invention is preferably carried out using a redox system of hydrogen peroxide, sodium peroxodisulfate and ascorbic acid. In a typical embodiment, these components are used at concentrations of 1 · 10 ⁻² mol% of hydrogen peroxide, 0.084 mol% of sodium peroxo disulfate, and 2.5 · 10 ⁻³ mol% of ascorbic acid.

The monomers used are preferably stabilized with commercially available polymerization inhibitors, more preferably with polymerization inhibitors acting only with oxygen, exemplifying hydroquinone monomethyl ethers.

Commercially available polymerization inhibitors are polymerization inhibitors which are used as storage stabilizers for each monomer for reasons of product safety. Examples of such storage stabilizers are hydroquinone, hydroquinone monomethyl ether, 2,5-di-tert-butylhydroquinone and 2,6-di-tert-butyl-4-methylphenol.

The reaction is preferably carried out in an apparatus which is also suitable for spray drying. Such reactors are described, for example, in K. Masters, Spray Drying Handbook, 5th Edition, Longman, 1991, page 23 to 66.

The reaction is preferably carried out in an apparatus in which the monomer solution can free fall in the form of monodisperse droplets. For example, the device described in US Pat. No. 5,269,980 is suitable for this purpose.

The diameter of the droplets produced in the spraying process is advantageously 50 to 1000 μm, preferably 100 to 600 μm in the present invention.

The reaction can be carried out at superatmospheric and low atmospheric pressure, preferably the reaction pressure that occurs when the offgas system is exposed to the atmosphere is preferred.

The reactor is purified by a flow of inert gas, preferably nitrogen, flowing through the reactor. Cocurrent operation is preferred. That is, the inert gas flows downstream through the reactor. The water vapor content of the inert gas is generally at most 1% by volume, preferably at most 0.5% by volume. The velocity of the inert gas is preferably adjusted such that the flow in the reactor is laminar, for example 0.02 to 1.5 m / s, preferably 0.05 to 0.4 m / s.

The inert gas is advantageously preheated to a reaction temperature of 90 to 300 ° C., preferably 150 to 210 ° C. upstream of the reactor.

The reaction emitter may be cooled, for example, in a heat exchanger. Water and unconverted acrylic acid condense in the process. The offgas can then be at least partially reheated and returned to the reactor as recycled gas. Preferably, the outlet gas is cooled such that the cooled outlet gas has the desired water vapor fraction for the reaction. Part of the exhaust gas can be removed from the system and replaced with fresh inert gas.

Particular preference is given to energy integration systems in which part of the heat released during cooling of the discharge gas is used for heating the recycle gas.

The reactor is preferably trace heated. Trace heating is preferably adjusted so that the wall temperature exceeds the reactor internal temperature at least 5 ° C. and condensation on the reactor wall is sufficiently prevented.

The reaction product can be removed from the reactor in a typical manner, preferably via a transfer screw, at the bottom and optionally dried to the desired residual moisture content and the desired residual monomer content.

The process of the present invention provides a water expandable polymer having high absorbency and low extractables.

Unless stated otherwise, parts are by weight. The dried hydrogels were tested using the following test methods standardized by the European Disposables and Nonwovens Association (Edana).

Centrifuge Retention (CRC): Edana Recommended Test Method No. 441.2-02

Extractables: Edana Recommended Test Method No. 470.2-02

<Examples 1 to 8>

An aqueous monomer solution consisting of partially neutralized acrylic acid and methylenebisdiacrylamide crosslinker was immediately mixed with a 0.1 wt% aqueous solution of 2,2′-azobis-2-amidinopropane dihydrochloride initiator upstream of the reactor.

The amount of crosslinking agent was 0.3% by weight based on the monomers used. The amount of initiator was 0.11% by weight based on the monomer solution used. The degree of neutralization of acrylic acid used was 75 mol%.

The reactor used was a special steel tube with a vertical length of 2,600 mm and a diameter of 164 mm. The temperature could be controlled through three independent heating circuits. A glass tube 102 mm in diameter was pushed from the top into the steel tube. A modified SBG-2000 vibrating orifice aerosol generator from Pallas GmbH, Karlsruhe, Germany, was placed on a glass tube. The vibrating orifice was 75 μm in diameter. A vibrating orifice aerosol generator was used to meter 0.4 l of nitrogen per minute and 1.0 ml of monomer solution per minute in the reactor. In addition, 10 l of nitrogen per minute preheated to the reaction temperature was metered in the space between the steel and glass tubes. The space was filled with Raschig rings and the lower end was opened for nitrogen. The Rasihi ring served as a flow straightener.

The product was collected at the bottom end of the reactor, dried at 125 ° C. for 1 hour and analyzed. Relative values were recorded based on the comparative examples.

Claims (8)

The spray polymerization method in the inert atmosphere of the monomer solution containing the free radically polymerizable monomer whose water content of a monomer solution is 55 weight% or more. The method of claim 1 wherein the water content of the monomer solution is at least 65% by weight. The method of claim 1 wherein the water content of the monomer solution is at least 70% by weight. The process according to any one of claims 1 to 3, wherein the reaction temperature is 90 to 300 ° C. 5. The process according to claim 1, wherein the free radically polymerizable monomer is acrylic acid and / or methacrylic acid. The method of claim 5 wherein the acrylic acid and / or methacrylic acid are neutralized at least 40%. The process of claim 1, wherein the free radically polymerizable monomer is a mixture of acrylic acid and potassium acrylate. A water expandable polymer obtainable according to the method of any one of claims 1 to 7.